In the past wells would get sequentially perforated and fractured generally in an uphole direction. Already treated zones would be isolated as the next uphole zone would be perforated and then fractured. A production packer would be located above all the zones and the production tubing would be tagged into the production packer before the well would be put on production. At some point in time the production rate from the well would be unsatisfactory and typically the well would be plugged and abandoned.
Over time and as an alternative to plugging and abandoning the well it was thought to re-fracture the well but the presence of the production packer with its limited drift dimension through the mandrel inhibited the idea from taking hold. The reason was that the small drift would limit the size of the service string that could be used to run in the bridge plug and perforating gun that would then also be needed to perform the refracturing after the reperforating. It simply was not possible to pump at the frac rates needed of about 50 barrels a minute or more through small diameter tubing that could allow the bridge plug to clear the drift of the production packer mandrel. In essence the surface pumping horsepower requirements would be impractical or the working pressures and fluid velocities in the pumping system would be so high as to reach pipe working pressure limits or create serious erosion issues from high fluid velocities.
The present invention addresses these and other issues to enable the use of larger drift components to alleviate the drift issue that previously limited the frac service string size. Additional features are provided to isolate low pressure rated casing from overpressure during refracturing. Locating devices can be used to pinpoint the perforation locations. Using swellable packers reduces the running in drift of the assembly as well as making it possible to remove the swelling packers after cutting loose an anchor packer positioned below the lowest zone to be perforated and refractured. The frac ports in the service string are located close to the bridge plug to reduce the sand buildup on the barrier that isolates already treated zones from those to be treated. These and other features of the present invention will be more readily apparent to those skilled in the art from a review of the details of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined from the appended claims.